The most commonly used cast aluminum alloys in structural applications in automotive and other industries include the Al—Si family of alloys, such as the 200 and 300 series aluminum alloys. They are used predominantly for their castability and machinability. In terms of castability, low silicon concentration has been thought to produce inherently poor castability. Similarly, although Al—Cu alloys have been developed for high strength applications, they have suffered from poor castability because of a severe hot tearing tendency.
In Al—Si casting alloys (e.g., alloys 319, 356, 390, 360, 380), the strengthening is achieved through heat treatment after casting, with addition of various alloying elements including, but not limited to, Cu and Mg. The heat treatment of cast aluminum involves at least a mechanism described as age hardening or precipitation strengthening. Heat treatment generally includes at least one or a combination of three steps: (1) solution treatment (also defined as T4) at a relatively high temperature below the melting point of the alloy, often for times exceeding 8 hours or more to dissolve its alloying (solute) elements and to homogenize or modify the microstructure; (2) rapid cooling, or quenching into a cold or warm liquid medium after solution treatment, such as water, to retain the solute elements in a supersaturated solid solution; and (3) artificial aging (T5) by holding the alloy for a period of time at an intermediate temperature suitable for achieving hardening or strengthening through precipitation. Solution treatment (T4) serves three main purposes: (1) dissolution of elements that will later cause age hardening, (2) spherodization of undissolved constituents, and (3) homogenization of solute concentrations in the material. Quenching after T4 solution treatment retains the solute elements in a supersaturated solid solution (SSS) and also creates a supersaturation of vacancies that enhances the diffusion and the dispersion of the precipitates. To maximize the strength of the alloy, the precipitation of all strengthening phases should be prevented during quenching. Aging (T5, either natural or artificial aging) creates a controlled dispersion of strengthening precipitates.
The addition of strengthening elements, such as Cu, Mg, and Mn, can have a significant effect on the physical properties of the materials. It has been reported that aluminum alloys with a high copper content (about 3-4%) have experienced an unacceptable rate of corrosion, especially in salt-containing environments. Typical high pressure die (HPDC) aluminum alloys, such as A 380 or 383, which are used for transmission and engine parts, contain 2-4% copper. It can be anticipated that the corrosion issue of these alloys will become more significant, particularly when longer warranty time and higher vehicle mileages are required.
FIG. 1 shows a photograph of an aluminum transmission cover which has corroded. FIG. 2 is a photograph showing pitted surface cavities due to presence of Q phase 10 (Al5Cu2Mg8Si5).
Although there is a commercial alloy 360 (nominal composition by weight: 9.5% Si, 1.3% Fe, 0.3% Mn, 0.5% Cu, 0.5% Mg, 0.5% Ni, 0.5% Zn, 0.15% Sn and balance Al) designated for corrosion resistance applications, this alloy may experience thermal fatigue problem over time in service, especially in high performance engine applications. Similar problems may occur with the alloy describe in U.S. Pat. No. 6,733,726.
Therefore, there is a need for improved castable aluminum alloys and for methods of making them.